The present invention relates to a thin film piezoelectric actuator and more particularly to a thin film piezoelectric actuator which can be favorably applied to a micro-electro-mechanical system (MEMS), can be easily controlled, and uses a piezoelectric thin film driven at a low voltage over a wide range.
In a microswitch or a variable capacitor by a conventional MEMS art, electrostatic force is mainly used as driving force. The MEMS actuator using the electrostatic force has an advantage that a very simple operation mechanism only applying a driving voltage to a pair of electrodes across a space is preferably used (for example, “Smart Structure and Materials 2002: Smart Electronics, MEMS and Nanotechnology”, V. K. Varadan, Editor, Proceedings of SPIE Vol. 4700 (2002), pp. 40-49).
On the other hand, from the nature that electrostatic force is inversely proportional to the square of a distance, when the spacing is up to about ⅔ of the initial spacing where the relation of the applied voltage and the movement distance of electrons is non-linear, a phenomenon called “pull-in” that the spacing is discontinuously closed is generated. Therefore, the driving range is narrow and to drive at a distance of longer than 1 μm, a driving voltage of higher than 20 V is generally necessary. At present, from the problem that the driving voltage is high, it is not easily applied to general consumer appliances.
In the conventional piezoelectric actuator, as piezoelectric ceramics, a PZT series (lead zirconate titanate) material having a very high piezoelectric property can be used, so that piezoelectric actuators in various shapes can be produced. However, to produce a highly efficient piezoelectric thin film of PZT series by the thin film art, problem arise that in a lead series material, the melting point thereof is low and the vapor pressure thereof is high, and the composition control in the film deposition process is difficult, and the lead series material cannot be used in a general semiconductor manufacturing line.
Therefore, the inventors of the present invention variously examined piezoelectric thin films which can be produced by the thin film deposition method and found that a film of aluminum nitride (AlN) or zinc oxide (ZnO) orientated in the c-axis having a wurtzite crystal structure is suitable. However, AlN and ZnO, compared with the aforementioned PZT family, have very low piezoelectric properties such as less than 1/10. Therefore, when preparing a piezoelectric actuator structure, to increase the displacement amount, it is necessary to thin and lengthen the actuator, that is, increase the aspect ratio. When a thin film having a structure of such a high aspect ratio is prepared, the cantilever beam structure often used in a piezoelectric actuator using piezoelectric ceramics is unstable structurally. Therefore, a double-clamped beam structure is more suitable.
FIG. 42 is a schematic view showing the sectional structure of a thin film piezoelectric actuator having the double-clamped beam structure examined by the inventors in the process of the present invention. The actuator includes a driving part 605 composed of a piezoelectric body 604 which is in contact with anchors 601 on both sides and is held by two sets of upper and lower electrodes 602 and 603 and a support film 606 for forming a bimorph structure. In this concrete example, the device is applied to a capacitive microswitch and at a central part 607 held by the two driving parts 606, switching electrode contacts 608 and 609 are provided.
The operation of the thin film piezoelectric actuator having a double-clamped beam structure is displayed in FIG. 43. The actuator applies a driving voltage to the two driving parts 605 prepared on both sides in the same direction, deforms them in a convex shape, and drive them. At this time, the central part 607 is inversely deformed in a concave shape and the switching electrode makes contact with it to set a switch ON state. At this time, in order to deform the central part 607, an excessive driving voltage is needed to be applied (first problem).
When the thin film piezoelectric actuator having the double-clamped beam structure is applied to a variable capacitor, at the central part 607, a first electrode of the variable capacitor and a second electrode opposite to it are provided. Also in a case of the variable capacitor, similarly to the capacitive microswitch, the thin film piezoelectric actuator is operated as shown in FIG. 43 and changes the spacing between the capacitor electrodes and changes the capacity. However, in a case of the variable capacitor, the central part is bent, so that it is difficult to vertically move the substrate and flat electrodes in a state that they are kept parallel with each other (second problem).
As explained above, in the conventional MEMS actuator, a problem arises that in the applied device, the driving voltage is high and the operation is not linear, and in the device designed to solve the problem, another structural problem is imposed.